Power scaling of a laser refers to increasing a laser's output power without substantially changing the geometry, shape, or principle of operation. Power scalability is considered an important advantage in a laser design. Usually, power scaling requires a more powerful pump source, stronger cooling, and an increase in size. It may also require reduction of the background loss in the laser resonator and, in particular, in the gain medium. One such approach for achieving power scalability is referred to as a “MOPA” (Master Oscillator Power Amplifier). A MOPA includes a master oscillator (MO) e.g., a stable, low power laser source producing a highly coherent beam, which provides an input, or seed to an optical power amplifier (PA). The optical PA increases the power of the “seed” beam, while generally preserving its main properties. Consequently, the MO has no need to be powerful, and no need to operate at high efficiency because the efficiency is determined largely by the PA. The MO is normally not used as a standalone entity, because of its low output. However, by seriesing multiple laser diodes in a light emitting array (i.e., 5 10, or more diodes) to pump a single gain medium; a “power oscillator” (PO) is created. The PO is conceptually the same as a MO; but with significantly more laser light output power. The PO is nothing more than a high power MO that is suitable for medium power applications like near earth range finding. The PO usually has a smaller output than a MOPA. A MOPAPA can be created in which the 1st PA creates seed light for the 2nd PA. By continually adding more and larger PAs to the chain kilowatt or even megawatt laser outputs are possible.
Generally, optical PAs include a gain medium. The gain medium comprises a host material which contains a particular concentration of dopant ions. An optical pumping source (e.g., a laser diode array) excites dopant ions of the gain medium to a higher energy state from which they can decay, via emission of a photon at the signal wavelength back to a lower energy level. Photonic emission may be spontaneous or stimulated, in which such transition of a dopant ion is induced by another photon. Preferably, pumping of the gain medium is sufficient to achieve a population inversion, in which more ions exist in an excited state than a lower energy state. Stimulated emission is induced within the gain medium by incoming light introduced in the form of a seed beam. Example structures include doped optical fiber waveguides, rods, slabs, and planar waveguide.
Pumping such optical systems generally requires a substantial amount of energy. For example, when such pumping is accomplished using laser diodes, the diodes are driven at current levels that can reach into the hundreds of Amperes. Laser drive currents for pumping a gain medium can be both single pulse and periodic in nature. Typically, the pulses are provided periodically, for short durations, followed by an off or no current period. Suitable laser diode currents for pumping MOs and PAs can be provided by laser diode driver circuits. Traditionally, in such MOPA configurations, two fully independent current driver circuits are generally provided, one for the PA laser diode array and another for the MO laser diode array. Each current driver circuit generally contains its own separate charge source, such as a storage capacitor. In operation, such current driver circuits are configured to provide rectangular current pulses (i.e., on/off, current/no current).